r hand
or when dust sticks to the ceiling; while cohesion is the clinging
together of the particles of the same substance, like the holding
together of the particles of your chair or of this paper.

When you put your hand into water it gets wet because the adhesion
of the water to your hand is stronger than the cohesion of the
water itself. The particles of the water are drawn to your hand more
powerfully than they are drawn to each other. But in the following
experiment, you have an example of cases where cohesion is stronger
than adhesion:

EXPERIMENT 16. Pour some mercury (quicksilver) into a small
dish and dip your finger into it. As you raise your finger,
see if the mercury follows it up as the water did in
Experiment 14. When you pull your finger all the way out, has
the mercury wet it at all? Put a lamp wick or a part of your
handkerchief into the mercury. Does it draw the mercury up as
it would draw up water?

[Illustration: FIG. 23. The mercury does not wet the finger, and as
the finger is lifted the mercury does not follow it.]

The reason for this peculiarity of mercury is that the pull between
the particles of mercury themselves is stronger than the pull between
them and your finger or handkerchief. In scientific language, the
cohesion of the mercury is stronger than its adhesion to your finger
or handkerchief. Although this seems unusual for a liquid, it is what
we naturally expect of solid things; you would be amazed if part of
the wood of your school seat stuck to you when you got up, for you
expect the particles in solid things to cohere--to have cohesion--much
more strongly than they adhere to something else. It is because solids
have such strong cohesion that they are solids.

_APPLICATION 13._ Explain why mercury cannot wet your fingers;
why rain falls in _drops_; why it is harder to drive a nail
into wood than into soap; why steel is hard.

INFERENCE EXERCISE

Explain the following:

51. Ink spilled on a plain board soaks in, but on a varnished
desk it can be easily wiped off.

52. When a window is soiled you can write on it with your
finger; then your finger becomes soiled.

53. A starched apron or shirt stays clean longer than an
unstarched one.

54. When you hold a lump of sugar with one edge just touching
the surface of a cup of coffee, the coffee runs up the lump.

55. A drop of water on a dry plat